Pressure-powered pumps have shown their worth on draining condensate from systems where centrifugal pumps have experienced reliability issues due to cavitation. This can be also be true on vacuum systems where having sufficient head on an electric pump can prove difficult. However, vacuum systems present their own unique challenges to pressure-powered pumps due to the large specific volume of the steam and cooler condensate temperatures.
At deep vacuum, the orifices in the vent and motive valves can be too small to allow for proper operation. The failure of the pump to vent (i.e., exhaust off) the motive pressure fast enough will result in slow cycling and reduced capacity because the pump cannot refill until the motive steam is exhausted. When starting to pump, the inflow of motive steam must be sufficient to build pressure and push the condensate out.
As the condensate temperature cools, the pump also becomes cooler and condenses the motive steam making it harder to build pressure. To stop condensate from filling the tank, the pressure inside the pump needs to be greater than the pressure in the fill line so the inlet check valve will close. If it fails to build pressure fast enough, the pump (tank) can continue to fill and flood. If a pressure-powered pump fills to the top, the pressure cannot push the condensate out of the tank which can cause the pump to stall. Backing up condensate from slow venting will increase pressure on the fill line requiring more motive pressure to build up before condensate flow into the tank stops, increasing the likelihood that the pump will flood and stall.
For example, consider a conventional float-operated pump with the vent (exhaust) and motive valve of typical size for a pressure-powered-pump. The orifice size is limited by the amount of buoyancy the float can generate which is limited by the size of the pump body to contain the float. While a larger float could operate larger valves, it would require a large pump body. So, what do you do when there isn’t enough exhaust or motive pressure to operate smoothly at very low levels of vacuum?
One solution to this challenge would be to go away from the float operated valves inside the pump and replace them with larger external valves. Although it may sound complicated, it can be done quite simply with a replacement probe head, control box with relay, and an external 3-way valve. Your new hero is a “Float Free” replacement head for the pump. This solution offers a valve with much larger openings providing faster exhausting for filling and more rapid pressure buildup for pumping. In application, this setup has provided superior performance compared to float operated pressure-powered pumps down to 28 plus inches of Mercury vacuum.